Physical vapor deposition (PVD) process has been widely used in semiconductor processing for the deposition of metal layers. A basic sputtering process frequently employs a plasma of argon gas which can be advantageously generated by a flow discharge. The argon plasma sustained by the secondary electrons generated from ion bombardment of a cathode. The charged ions in the argon plasma then defuse into a special zone between the cathode and the anode to acquire a higher energy level and to strike the cathode, or the target surface. The momentum of the argon ion transverse to the target material and ejects one or more atoms from the surface of the target. The ejected atom of a neutral charge flies through the plasma and lands on a wafer surface.
In the sputtering deposition process, one most important concern is to increase the ion bombardment rate on the cathode such that a reasonable deposition rate can be achieved. Since the glow discharge relies on the secondary electrons from the target and therefore is intrinsically inefficient, it is desirable to design means to increase the secondary electron production and the efficiency of ionization in order to improve the sputtering rate. One of such designs is the use of an E.times.B field to enhance the trapping of electrons. The cathode design using such a magnetic field is known as magnetron. The improved magnetron sputtering process increases the sputtering deposition rate and makes sputtering a leading technique in physical vapor deposition.
In a magnetron sputtering system, axial magnetic field is utilized with a planar diode for increasing the path length of the electrons and furthermore, to keep them away from the chamber walls. The object is to trap electrons near the target to increase their ionizing effect and thus increasing the deposition rate. The electric field and the magnetic field generated are usually perpendicular to each other. The magnetic field, when utilized in a magnetron sputtering technique, captures and spiral electrons to increase their ionizing efficiency in the vicinity of the sputtering target. In the magnetron sputtering of aluminum metal, deposition rates as high as 1 .mu.m/min have been achieved.
FIG. 1 shows a simplified cross-sectional view of a typical magnetron sputtering system 10. The metal target material to be sputtered is normally made into a disc 12 that is thermally bonded to the cathode 14. A large amount of power is supplied to the argon plasma 16 to maximize the sputtering rate of metal particles 20 from the target 12. Since most of the power is absorbed by the sputtering target 12, the target must be cooled through thermal contact 22 with the cathode 14 which in turn is water-cooled through a cooling water supply inlet 26. A wafer 28 is positioned on a heater 30, which is also a wafer platform in the sputtering chamber 18. After the chamber 18 is first pumped through a pump outlet 24, argon gas is fed into chamber 18 through a plasma gas inlet 32. Ceramic insulators 34 are further provided to electrically insulate the cathode 14 from the chamber wall 36.
In addition to the high deposition rate requirement for a sputtering apparatus, another critical criterion for a sputtering deposition process is its ability to produce a deposited film with high uniformity. This is especially critical when large wafers, i.e., wafers larger than 200 mm diameter, are being deposited in a metal sputtering chamber. To achieve both the high deposition rate and the film uniformity, more recently developed sputtering machines are equipped with cathodes that have rotating permanent magnets 40 that are made of rare earth, high strength materials.
A cathode 14 which rotates behind a metal target 12 in the rotational direction as marked is shown in a plane view in FIG. 2. Since the magnets 40 are in a fixed position in the radial direction of the cathode 14, an inherent drawback of the cathode is its inability to produce a magnetic flux field that has uniform flux distribution. As a result, certain areas on the target surface 38 is bombarded more than other areas. This is caused by the non-uniform plasma ion distribution in the plasma cloud 16 which is in turn caused by the non-uniform magnetic flux distribution formed during the rotation of the cathode 14.
In the conventional magnetron sputter, the magnets are permanently mounted in a cathode and rotates above a metal target (as shown in FIG. 1) to create a magnetic flux field. In order to produce the required magnetic strength and flux distribution, different magnet assemblies are required for use in different processes. This requires a tedious and labor intensive task of replacing cathodes in a magnetron sputter apparatus frequently.
A typical example of a target erosion wherein certain areas in the surface of a metal target suffer more severe plasma ion bombardment resulting in more severe erosion for a titanium target is shown in an erosion profile in FIG. 3A. When such a severe, non-uniform erosion profile is formed on a metal target, the target must be replaced more frequently than normally necessary in order to avoid the wearing-through of certain areas on the target by the non-uniform bombardment. As seen in FIG. 3A, the titanium target is almost worn through 2/3 of the way at an outer fringe area 44 (shows up in a donut form when viewed from the top of the target) when compared to the center area 42 of the titanium target. A similar graph illustrating the non-uniform wear of a TiN target is shown in FIG. 1B.
The non-uniform wear, or consumption of the metal target surface shown in FIGS. 3A and 3B not only causes a premature failing and a need for replacement of the metal target, but also severely affects the uniformity of the deposited film. This is shown in FIG. 4, a plot of non-uniformity or deviation of the film thickness against time for the sputter deposition of TiN films. At points A and B, new targets were installed to replace a prematurely worn target. As seen in FIG. 4, of the non-uniformity of the deposited film gradually increases at approximately the same rate each time after the new target is installed. The data of FIG. 4 can be coordinated with the data of FIG. 3B since the more severely eroded TiN target (i.e., having more severely formed peaks and valleys), the more non-uniform the magnetic flux distribution resulting in greater non-uniformity in the deposited TiN film. It is seen in FIG. 4, at the beginning of a new target, the non-uniformity of the deposited TiN film is very small. The non-uniformity gradually increases as the surface of the metal target is more severely eroded forming donut sections which are bombarded more severely by the plasma ions due to the non-uniform magnetic flux distribution. The non-uniform magnetic flux distribution, as previously discussed, is cased by a cathode that has magnets mounted on top in fixed positions.
It is therefore an object of the present invention to provide a magnetron assembly for sputter deposition that does not have the drawbacks or shortcomings of the conventional magnetron assemblies.
It is another object of the present invention to provide a magnetron assembly that is equipped with traversing magnets capable of making linear motion in a radial direction simultaneously with the rotational motion of the assembly.
It is a further object of the present invention to provide a magnetron assembly for sputter deposition that is equipped with traversing means for mounting the magnets which is driven by motors capable of producing a substantially uniform magnetic flux distribution.
It is another further object of the present invention to provide a magnetron assembly that is equipped with traversing magnets capable of making linear motions at a speed in a range between about 2 mm/sec and about 20 mm/sec.
It is still another object of the present invention to provide a method for sputter depositing a metal on an electronic substrate that is capable of depositing a more uniform film on the substrate.
It is yet another object of the present invention to provide a method for sputter depositing a metal on an electronic substrate by mounting magnets on a traversing means and moving the magnets in a radial direction while simultaneously rotating the magnetron assembly.
It is still another further object of the present invention to provide a sputter deposition chamber that is equipped with a magnetron assembly which has magnets mounted on traversing means for making radial movements when the assembly rotates around a center axis.
It is yet another further object of the present invention to provide a sputter deposition chamber equipped with a magnetron assembly formed of a disc for rotating about a center axis and magnets mounted on traversing means for moving toward and away from the center axis simultaneously with a rotational motion of the assembly.